detect duplicate local vars

lox/interpreter.janet

···
     left right))
 
 (defn- declare-var [{:token [_ name]}]
+  (when (table/rawget (curenv) name)
+    (errorf "Already a variable named '%s' in this scope." name))
   (setdyn name @[]))
 
 (defn- define-var [{:token [_ name]} val]
···
                        (when init (set-var name val)))
     [:fun name params body]
     (let [arity (length params)
-          env (table/clone (curenv))
+          closure (table/clone (curenv))
+          env (table/setproto @{} closure)
           call (fn [args]
                  (loop [i :range [arity]
                         :let [name (params i) arg (args i)]]

readme.md

···
  - [x] Control Flow
  - [x] Functions
  - [ ] Resolving and Binding
+   - [x] Lexical scope
+   - [ ] Prevent self-referential initializer
+   - [x] Prevent variable name reuse in same scope
+   - [ ] Prevent top-level return
  - [ ] Classes
  - [ ] Inheritance

test/test_main.janet

···
 (test-error (process "fun add(x) { return x + 1; } print add();") "Expected 1 arguments but got 0.")
 (test-error (process "fun add(x) { return x + 1; } print add(1, 2);") "Expected 1 arguments but got 2.")
 (test-error (process "var x = 1; x();") "Can only call functions and classes.")
+(test-error (process "var x = 1; var x = 2; print x;") "Already a variable named 'x' in this scope.")